The present invention relates generally to communication systems, and more particularly, to a differential phase shift keyed demodulator system for communication systems.
Communication systems are in widespread use and take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of modulating the information signal into a form suitable for transmission over the channel. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to receive and then to demodulate the received signal so as to produce an estimate of the original information signal.
Analog and digital transmission methods are used to transmit an information signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of information signals, improved security of communication through the use of encryption, and increased capacity.
To transmit an information signal (either analog or digital) over a bandpass communication channel, the information signal must be manipulated into a form suitable for efficient transmission over the channel by modulating the information signal. Modulation involves varying some parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. At the receiver point of the communication channel, a receiver re-creates the original information signal from a degraded version of the transmitted signal by a process known as demodulation.
A Mach-Zehnder interferometer (MZI) is commonly used in a differential phase shift keyed (DPSK) demodulator to convert the phase modulation of a signal into an amplitude modulation for subsequent detection. The principal technical challenge associated with this approach is maintaining a stable output from the MZI in the presence of thermal and acoustic perturbations. Specifically, in order for the MZI to function properly, the two optical signals that interfere at the MZI output must be co-polarized (as they were at the entry to the MZI), and their only phase difference must be due to the DPSK phase change. The environmental factors mentioned above can affect both the polarization and phase of the signals, thereby distorting the MZI output that is detected.
The disadvantages associated with this conventional DPSK demodulation technique have made it apparent that a new technique for DPSK demodulation is needed. The new technique should eliminate the polarization problem in prior art DPSK demodulation systems caused by thermal and acoustic perturbations. Additionally, the new technique should reduce assembly complexity allowing lower cost and greater reliability. The present invention is directed to these ends.
It is, therefore, an object of the invention to provide an improved and reliable differential phase shift keyed demodulator system. Another object of the invention is to eliminate the polarization problem in prior art DPSK demodulation systems caused by thermal and acoustic perturbations.
In accordance with the objects of this invention, a differential phase shift keyed demodulator system is provided. In one embodiment of the invention, a differential phase shift keyed demodulator system 18 uses a Michelson interferometer configuration in which polarization maintenance is accomplished by using a single 50:50 coupler 28 coupled to a first Faraday rotator mirror 30 and a second Faraday rotator mirror 32 through a first fiber 34 and a second fiber 36, respectively. In this way, system 18 passively eliminates the polarization problem associated with using a Mach-Zehnder interferometer.
The present invention thus achieves an improved differential phase shift keyed demodulator system. The present invention is advantageous in that it reduces assembly complexity, thereby allowing lower cost and greater reliability.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.